1. Field of the Invention
The present-invention relates to a fluid based accelerometer and inclinometer, and more particularly to a transducer which determines acceleration, inclination, position or velocity based on a temperature differential caused by the effect of acceleration on free or natural convection.
2. Description of the Related Art
Accelerometers and inclinometers of various types have been used in many different applications including robotics, transportation, consumer electronics and toys. A variety of mechanical, and electrical devices are available for measuring acceleration and inclination including piezoelectric, piezoresistive, force balanced and capacitive accelerometers. However, it is desirable to provide a highly sensitive accelerometer having a smaller size and lower cost than is available in these known accelerometers.
Piezoelectric accelerometers include a piezoelectric or crystalline material which generates an emf in response to acceleration. Piezoelectric accelerometers are used primarily in the measurement of vibration and are generally not capable of measuring constant acceleration. Examples of piezoelectric accelerometers are disclosed in U.S. Pat. Nos. 5,235,237 and 5,193,392.
Piezoresistive accelerometers include mechanical structures which generate strain in critical locations due to acceleration. The acceleration is measured by placing piezoresistors in the locations where strain occurs to generate electric signals corresponding to the acceleration. Piezoresistive accelerometers are capable of measuring constant accelerations at high accelerations, at a moderate cost. However, piezoresistive accelerometers have the disadvantage that they can be used only in a limited temperature range and are sensitive to changes in environmental temperature. An example of a piezoresistive accelerometer is disclosed in U.S. Pat. No. 5,277,064.
In a force balanced or servo accelerometer a mass is spring-suspended between two permanent magnets. A displacement of the mass due to acceleration is sensed by a capacitive or other probe. A signal from that probe is amplified and the resulting current passes through a coil wound on the mass, producing a rebalancing force that restores the mass to its original position. Force balanced or servo accelerometers provide high sensitivity and precision, but at a very high cost. An example of a servo accelerometer is disclosed in U.S. Pat. No. 3,664,196.
Capacitive accelerometers include parallel plates which move closer together in response to acceleration. The capacitance between the parallel plates can be measured with electronics. A capacitive accelerometer can be made in a relatively small size. However, as the capacitive accelerometer is made smaller, the size of the electronics required to sense and measure the small output increases. Therefore, the overall size and cost of the capacitive accelerometer is about the same as that of the piezoresistive accelerometers. Examples of capacitive accelerometers are disclosed in U.S. Pat. Nos. 5,303,589 and 5,243,861.
Accelerometers which are also capable of measuring inclination or static position are called inclinometers. An inclinometer measures the angle of the force of gravity on an object to determine its position relative to the earth. Inclinometers such as the one disclosed in U.S. Pat. No. 5,092,171, measure inclination by measuring the angle of the surface of a fluid with respect to the container for the fluid.
Another type of fluid based measuring device is an angular rate sensor which measures angular rate change, rather than measuring acceleration. The operating principle of the fluidic angular rate sensor is that a gas jetted from a nozzle is biased in the direction opposite the direction of acceleration. In the fluidic angular rate sensor a pair of sensing wires are positioned so that the jet of gas equally contacts both wires when the device is stationary. If an angular rate change occurs, the gas jet is biased to one direction so that one of the sensing wires is cooled more than the other. The difference in the resistances of the two sensing wires is measured to determine the angular rate change. This type of angular rate sensor has the disadvantage that a pump is required to create the gas jet, making the device relatively large and expensive. Examples of such angular rate sensors are shown in U.S. Pat. Nos. 3,500,691, 4,951,507 and 5,012,676.